Polymeric films such as PET film are commonly used to transfer metal to paperboard and other substrates for use in packaging, greeting cards, and similar product applications where it is desirable to give the product a metallic appearance. The technique of transferring metal from film to another substrate is used where it is not practical to metalize such substrates directly.
One typical metal transfer technique uses a metalized conventional polymeric film and bonds the complete metalized film structure to the substrate. Typical methods for depositing metal onto the polymeric film include vapor and sputter metallization processes. Such deposition techniques typically create strong bonds between the metal and the film. Because metal does not readily separate from the polymeric layer of the film, in the transfer procedure the paperboard substrate becomes permanently bonded to the metalized polymeric film. This can create excessive non-recoverable scrap since the resulting laminate, for example generated by post-consumer packaging disposal, can not be easily recycled.
Another technique that is well known and practiced in industry is to create a carrier film that can be used to transfer a metal layer to paperboard and other substrate materials. The carrier film is created by coating a layer of release material on the base polymeric film in a secondary, separate process from producing the base polymeric film. Subsequently, and in an entirely separate third process, a metal layer is deposited onto the release layer for example by vapor or sputter metallization. The individual steps may be carried out at different locations by different converters. A disadvantage is that an extra processing step is required, in particular the step of applying a release layer. A need exists for biaxially oriented polyester films that can transfer metal from a layer deposited onto the carrier film surface to another substrate without the need for costly coatings between the metal layer and the carrier film surface.
The primary function of the release layer is to provide appropriate adhesion between the polymeric film surface and the metal layer. The adhesion of the metal to the film surface should be strong enough to endure handling in manufacture, packaging, shipping, etc. prior to metal transfer. However, adhesion should be sufficiently weak that the metal layer cleanly separates from the carrier film surface when contacted with the substrate.
There is a need for a method of metal transfer from a polymeric film to a substrate in which a metal layer can be applied directly onto the polymeric film and in which the metal layer readily releases and separates from the film. It is desirable to have a carrier film that does not have an added release layer on the surface of the polymeric film. It is further desired to have a carrier film for metal transfer in which the base layer is a single polymeric composition that can be recycled after the metal has been transferred. Still further it is desired to have a polyester-based carrier film and transfer film that is free of a non-polyester release layer.